Optical fibers, as used in communications, afford many advantages, such as, for example, large bandwidth capacity and small size, over the more common and typical metallic wire arrangements.
On the other hand, optical fibers, which are generally made of fused silica or other glasses, are extremely delicate and are detrimentally affected by tensile stresses, abrupt bending, and static fatigue, in addition to various other types of stress. Thus, glass fibers, despite a theoretical high tensile strength, in actuality have low tensile strength and are subject to breakage when incorporated in a cable that, as in outside plant applications, must be pulled through ducts or the like. When such a cable is bent to accommodate changes in routing, for example, the fibers contained therein, if bent too sharply, as often happens, are subject to microbending cracks and consequently loss of signal transmission capability.
If the cable is subjected to moisture, as almost invariably happens in outside plant applications, this too can have a detrimental effect on the optical fibers. In the presence of moisture, glass will ultimately fracture under sustained stress even though the stress is actually below the tensile strength of the fiber because of the growth of surface flaws, which are aggravated by the moisture. Furthermore, glass fibers, in very long lengths, exhibit a low strain at break, usually less than one-half of one percent elongation before fracture. Inasmuch as each fiber in a cable carries or transmits several signal channels, any fracture occurring in just one fiber can mean the loss of the channels transmitted thereon, as well as possibly or probably necessitating the replacement of the section of the cable wherein the fracture occurred.
In view of the foregoing problems, as well as others inherent in or arising from, the rise of optical fibers in cables, much effort has been directed toward a cable design which eliminates or at least minimizes the detrimental effects of the various kinds of stress, as noted in the foregoing. In U.S. Pat. No. 4,078,853 of Kempf, et al., there is shown a "loose tube" cable design that overcomes most of the stress problems. This desirable result obtains from a structure in which the optical fibers are structurally isolated from the surrounding loading or cable environment. According to the invention of that patent, a plurality of light transmitting optical fibers, forming a cable core, are contained in a loose-fitting envelope or jacket so that the core fibers are substantially longitudinally decoupled from the rest of the cable structure and thus are not subjected to longitudinal tensile forces applied to the cable itself. Such a structural isolation also minimizes radial impact loads on the fibers of the core. Surrounding the enveloping structure or inner jacket is an outer jacket which is reinforced with primary strength members having a tensile modulus and a strain at break greater than that of the glass fibers. The primary strength members are tightly coupled to, or integrated into, the outer cable jacket so that they will carry the expected loads. Hence, under special loading conditions, the externally applied tensile stresses are substantially taken up by the primary strength members and not passed on to its core fibers. The optical fibers within the loose tube or tubes, where a plurality of such tubes are included in the cable, are preferably longer than the tube itself, being coiled or otherwise helixed into the tube in a slack state. As a consequence, any bending or tensile elongations of the cable itself does not immediately affect the fibers, such stresses being countered or absorbed by the cable structure itself without being applied to the fibers as a result of their being de-coupled from the physical cable structure. As a result, the fibers are, in effect, protected from these stresses with a greatly decreased likelihood that they will be affected thereby.
In some cable configurations, each loose tube has a central organizing member extending along its length about which the fibers are helically wound, thus creating considerable fiber slack while holding the fibers in an organized arrangement. Such an arrangement enhances the protection of the fibers from not only longitudinal stresses but from bending stresses also. Inasmuch as the fibers in a loose tube arrangement, whether they be stranded or incorporated into a ribbon structure, are individually accessible, such an arrangement also lends itself to splicing, either of individual fibers, or of groups of fibers.
One of the most prevalent problems in cable use, regardless of whether the cable contains metallic conductors or optical fibers, is the intrusion of moisture into the cable. This problem is especially acute in outside plant cable use, where ambient conditions are constantly changing. These changes in the ambient conditions lead to the diffusion of moisture into the cable from the outside thereof, and such diffusion eventually can lead to an undesirably high level of moisture inside the cable, which can have a detrimental effect on the signal transmitting members, and hence, on the signal transmission. As pointed out hereinbefore, moisture detrimentally affects both metallic conductors and optical fibers. Moisture can also be introduced into the cable because of a compromise in the integrity of, for example, the outer jacket. Common causes of such failure of the jacket can be rodent attacks or mechanical impacts tending to damage the jacket. Whenever moisture is present in the interior of a cable, it tends, over a period of time, to migrate or flow longitudinally along the cable and into cable connections at the splice closures or terminals and the like. In the special case of optical fibers, passage of the water therealong to connection points or terminals and associated equipment located, for example, inside closures, can not only result in damage to such equipment, especially to any metal parts thereof, but can cause problems at low temperature or freezing environments due to fiber microbending.
Many optical fiber cables have one or more longitudinally extending tapes wrapped around the core of the cable which include a super-absorbent material that swells upon contact with water to block the flow of water along the cable length. In U.S. Pat. No. 4,867,526 of Arroyo, such a cable is disclosed. In the cable of that patent, there is interposed between the core and the jacket an elongated substrate member of non-metallic, non-woven, web-like material in the form of a tape which has been impregnated with a super-absorbent material. The tape material is relatively compressible and has sufficient porosity and sufficient super-absorbent capacity so that it functions as a water block upon contact therewith. In another prior an cable, a water blockable yarn is interposed between a core and an inner surface of the cable jacket. The yarn extends linearly along the cable or, alternatively, it may be helically wrapped about a portion of the sheath system. The yarn preferably is one which comprises a super-absorbent fiber material which, upon contact with water, swells and inhibits the flow or movement of water within the cable.
In other prior art arrangements, efforts have been made to use a yarn-like strength member that has been treated with a super-absorbent liquid material which, when dry, fills the interstices of the yarn-like strength member. In another embodiment, a filamentary strand material comprising a water absorbent fibrous material is wrapped around each of the strength members. It has been found that, for a number of reasons, treating the strength member with a super-absorbent material is both impractical and uneconomical.
In U.S. Pat. No. 5,133,034 of Arroyo et al., there is shown an optical fiber cable which overcomes most of the aforementioned problems. That cable comprises a plurality of optical fiber components, each of which includes a buffered fiber and a high strength aramid yarn. The components are arranged about a central organizer and are surrounded by a water blocking strength member system. The strength member system comprises a strength member layer disposed between two adjacent super-absorbent material layers. The entire assembly is surrounded by and enclosed within an outer jacket, preferably of a low halogen, fire resistant plastic material. Supplementary water blocking members of water swellable fiber material are disposed within the jacket to fill the gaps and interstices created by the fiber component arrangement.
Prior an cables, as evidenced by the arrangement discussed in the foregoing, have been directed primarily to buffered fiber or ribbon fiber arrangements and, for the most part, have not addressed the problem of water intrusion into a loose tube cable arrangement. In a loose tube arrangement, the tubes, themselves, are conduits for the passage of water therealong, and thus, present an additional problem not present in a typical buffered fiber arrangement as shown in the U.S. Pat. No. 5,133,034 patent. In general, it has been the practice to inject a flooding compound, such as petrolatum grease, into the cable sheath to act as a moisture barrier and, in some instances, to surround the flooding compound with a layer of aluminum tape. The flooding compound is of grease-like consistency, and must be removed from the fibers when splicing or termination operations are to be performed. Thus, it is common practice prior to splicing, for example, to use grease removing compounds and wipes to clean the fibers, a messy and time consuming operation. The aluminum tape simply compounds the problem inasmuch as it, too, must be removed. Removal of the aluminum tape can be especially difficult when it adheres to the inner jacket, as often happens. In addition, the aluminum tape, being metallic can, where the cable is used in outside plant applications, attract lightening.